Balanced devices operating at high frequencies including, but not limited to, microwave frequencies are becoming more and more prevalent in modem systems, especially communication systems. Concomitant with the use of such devices comes a need to measure a performance of the devices. As with single-ended (i.e., non-balanced) microwave devices, balanced microwave devices may be characterized using S-parameters. However, in the case of balanced devices, so-called ‘mixed mode’ S-parameters are generally used to characterize the device instead of the more conventional or single-ended S-parameters. Mixed mode S-parameters relate differential and common mode signals applied to ports of the balanced device to differential and common mode responses at the ports.
A multiport vector network analyzer (VNA) may be used to measure S-parameters of a multiport device. Since a balanced device may be viewed as a generalized multiport device, conceptually a multiport VNA may be used to measure the S-parameters of a balanced device. Unfortunately, most conventional multiport VNAs measure multiport devices having single-ended (i.e., non-differential) ports. That is, the multiport VNA applies a single-ended signal to a port of a device under test (DUT) and a single-ended response is measured at each of the ports of the DUT by the VNA. Once measured, the measured single-ended S-parameters are converted into mixed mode S-parameters for the balanced device using modal decomposition. Unfortunately, many balanced devices behave differently in response to a single-ended stimulus signal such as those generated by a conventional multiport VNA than to a true differential stimulus signal. Therefore, the single ended S-parameters measured by the conventional VNA may not accurately reflect a performance of the balanced device in the presence of a differential stimulus signal. In a worst case, the balanced device may even fail to operate (e.g., become unstable resulting in unwanted oscillations) or be damaged when presented with a single-ended stimulus signal.
To overcome the limitations of conventional multiport VNAs with respect to balanced device measurement, specialized VNA systems that produce and directly measure balanced signals have been proposed and even constructed. However, such specialized VNA systems either omit portions of the system during a calibration or employ specialized calibration standards and methods. Omitting portions of the system during calibration may lead to errors that are unacceptably high for many practical applications. Specialized calibrations standards may be difficult to construct and/or characterize with sufficient accuracy to support an acceptable level of calibration. Moreover, using specialized VNA systems with or without specialized calibration standards for measuring a balanced device often may be prohibitively expensive.
Accordingly, it would be advantageous to be able to measure a balanced device under test (DUT) with a true differential stimulus signal while still using a conventional single-ended multiport vector network analyzer. Moreover, it would be advantageous if a calibration associated with such balanced DUT measurement accounted for all error sources including any devices and/or test fixtures between the VNA and the DUT while still employing conventional calibration standards and methods. Such a way of producing calibrated measurements of a balanced DUT would address a long-standing need in the area of balanced device measurement at microwave frequencies.